SUMMARY

In nearly every procellariiform species, the sense of smell appears to be
highly adapted for foraging at sea, but the sense of smell among the diving
petrels is enigmatic. These birds forage at considerable depth and are not
attracted to odour cues at sea. However, several procellariiform species have
recently been shown to relocate their nesting burrows by scent, suggesting
that these birds use an olfactory signature to identify the home burrow. We
wanted to know whether diving petrels use smell in this way. We tested the
common diving petrel Pelecanoides urinatrix and the South-Georgian
diving petrel Pelecanoides georgicus to determine whether diving
petrels were able to recognise their burrow by scent alone. To verify the
efficacy of the method, we also tested a bird that is known to use olfaction
for foraging and nest recognition, the thin-billed prion Pachyptila
belcheri. In two-choice T-maze trials, we found that, for all species,
individuals significantly preferred the odour of their own nest material to
that of a conspecific. Our findings strongly suggest that an
individual-specific odour provides an olfactory signature that allows
burrowing petrels to recognize their own burrow. Since this ability seems to
be well developed in diving petrels, our data further implicate a novel
adaptation for olfaction in these two species that have been presumed to lack
a well-developed sense of smell.

Introduction

In procellariiformes, olfaction is used in two different ways: to help
birds locate prey items and to help birds locate their nesting sites. At-sea
experiments have demonstrated that these seabirds are attracted to
prey-related odours such as krill and cod liver oil
(Grubb, 1972;
Verheyden and Jouventin, 1994;
Lequette et al., 1989;
Hutchison and Wenzel, 1980).
Smaller procellariiformes, such as petrels and prions (Procellariidae), are
also attracted to dimethyl sulphide
(Nevitt et al., 1995;
Nevitt and Haberman, 2003), a
compound associated with primary production in the ocean (reviewed in
Nevitt, 2000).
Procellariiformes use their sense of smell to help locate dense regions of
prey within productive areas of the ocean.

Odours emanating from the burrow help birds locate the nest site when
returning under the cover of darkness. Arriving at night is critical to a
bird's survival since burrow-nesting petrels tend to be small and susceptible
to predation by skuas (Catharacta skua lönnbergi;
Stercorariidae; Mougeot et al.,
1998; Warham,
1996; Thoresen,
1969; Payne and Prince,
1979). Birds find their burrow at night despite poorly developed
night vision (Brooke, 1989;
Martin and Brooke, 1991),
suggesting that cues other than those obtained visually may be important. The
characteristic smell of an individual's burrow is thought to assist returning
birds in relocating their burrow quickly and efficiently among the thousands
of others (Thibault and Holyoak,
1978; Warham,
1996).

The ability of individuals to recognise their burrow by smell has been
established in several species of petrels, including storm petrels
(Hydrobatidae), Cory's shearwaters (Calonectris diomedea), blue
petrels (Halobaena caerulea) and prions (Pachyptila spp.)
(Grubb, 1973,
1974,
1979;
Benvenuti et al., 1993;
Minguez, 1997;
Bonadonna et al., 2001;
Bonadonna and Bretagnolle,
2002). Bonadonna and Bretagnolle
(2002), in particular,
highlighted that species returning to colonies at night need an intact sense
of smell to relocate their nest, whereas species that return during the day
relocate the nest even if they are experimentally rendered anosmic. It has
also been suggested that the olfactory signature of the nest is critical for
nest relocation [Antarctic prion (Pachyptila desolata),
Bonadonna et al., 2003; blue
petrels (Halobaena caerulea),
Bonadonna et al., in press).
Using T-maze techniques, these studies demonstrated that birds can
distinguish the odour of their own nest from that of a conspecific; however,
birds fail if odours are absent or if the sense of smell is blocked. Since
feathers and faeces are the primary bird-derived material found in the
burrows, these results collectively suggest that birds recognise a
burrow-specific odour for burrow recognition.

A group of burrow-nesting species that have not been studied with respect
to nest recognition are the diving petrels (Pelecanoididae). Researchers have
assumed that these birds have a poor sense of smell since they have relatively
small olfactory bulbs compared with other procellariiformes
(Bang and Cobb, 1968).
Specifically, the relative olfactory bulb ratios of 18, South-Georgian diving
petrels (Pelecanoides georgicus) and 23, common diving petrels
(Pelecanoides urinatrix) were the smallest measured in the
procellariiformes (Warham,
1996). Diving petrels also fail to recruit to prey-related odours
at sea (Nevitt et al., 1995;
Nevitt, 2000). Additionally, a
recent study showed that `sleeping' common diving petrel chicks do not respond
to prey-related (dimethyl sulphide) and novel (phenyl ethyl alcohol)
odourants, suggesting a lack of sensitivity to odourants, at least at an early
age (Cunningham et al.,
2003).

Our aim in the present study was to test the ability of two species of
diving petrels to distinguish the odour of their own nest. An additional
experiment on a species that uses olfaction both for foraging and for burrow
locating was also performed for comparison.

Materials and methods

Tests took place during the birds' incubation period in the Kerguelen
archipelago, southern Indian Ocean. We tested 22 common diving petrels
(Pelecanoides urinatrix Gmelin) and eight thin-billed prions
(Pachyptila belcheri Matthews) during December 2000 and January 2001
on Mayes Island (49°28′ S, 69°57′ E). 25 South-Georgian
diving petrels (Pelecanoides georgicus Murphy and Harper) were tested
in January and December 2002 on Verte Island (49°51′ S,
70°05′ E). The burrows of all species tested consist of an access
gallery and an incubating chamber. Burrows were fitted with an access window
over the incubating chamber to facilitate capture of the bird and inspection
of the nest during the experiments. When access was not needed, or during
testing, this access window was closed with a rock.

In both field seasons, the maze had the same design and consisted of a box
with two flexible, expandable, corrugated pipes (10 cm diameter) that were
separated from the central chamber by a removable divider
(Fig. 1). The first 20-30 cm of
the two pipes were placed perpendicular to the box wall to reduce a possible
visual effect on the choice.

Maze used to test thin-billed prions. In the case of common diving petrels,
one arm of the maze was not connected directly to a burrow but contained nest
material at the end. In the case of South-Georgian diving petrels, both arms
of the maze were not connected to burrows and contained nest material at the
end.

Birds were captured and kept in a cloth bag during the arrangement of the
maze. The subject's burrow was randomly assigned to either the left or the
right arm of the maze. After the maze was set up, the bird was put in the box.
Five minutes later, the divider was removed. The test ended when the bird
arrived at the end of one pipe or after 15 min if the bird did not choose. The
choice was assessed by the noise of the bird walking in the corrugated pipe.
The time elapsing between the removal of the divider and the bird's choice was
recorded. The protocol and the maze employed during the two field seasons had
small differences as follows.

Mayes Island: December 2000 and January 2001

All the birds were tested while the mate was at sea to forage. The maze's
central chamber was a cardboard box measuring 20 cm×40 cm×20 cm.
In the case of thin-billed prions, one pipe was connected to the nest of the
subject bird and the second pipe to the nest of a conspecific neighbour. The
owner of the neighbouring nest was kept in a cloth bag far from the maze
during the whole experiment. Consequently, both burrows were empty during the
trial. In the case of common diving petrels, the second pipe was connected to
a box containing material from another common diving petrel nest. This was
necessary due to the difficulty of finding diving petrels' nests within a
reasonable distance. The length of the two pipes was similar within each
single trial and ranged between 50 cm and 130 cm depending on the position of
the nests. Tests were performed in the night-time for prions and in the
daytime for diving petrels. In the latter case, the apparatus was covered with
a thin black blanket to keep the box and the pipes in the dark.

Verte Island: January and December 2002

All the birds were tested while the mate was at sea to forage. All tests
took place at night. The maze's central chamber was a wooden box of 20
cm×47 cm×20 cm. The difficulty of finding South-Georgian diving
petrels' nests within a reasonable distance obliged us to modify the
experimental protocol for this species. All the experiments were performed at
least 4 m from the tested bird's nest, and both 30 cm-length pipes were closed
at the end with a metal dish containing nest material (from the tested bird
and from a conspecific neighbour).

Results

Most of the tested diving petrels did not choose an arm of the maze (up to
60% of common diving petrels and up to 48% of South-Georgian diving petrels;
Table 1), whereas all of the
thin-billed prions did choose. Diving petrels that did not choose were
inactive after removal of the divider, remaining immobile inside the main
central chamber throughout the experiment. To compare frequencies of choices
between the two species of diving petrels we considered three types of choice:
own nest, neighbour's nest and no-choice. We found no significant differences
in the frequencies of the three choices (likelihood ratio χ2
test, P=0.15). When a choice was made, both species of diving petrel
preferred the arm of the maze issuing odours from their own nest (binomial
test, own nest vs neighbour's nest: South-Georgian diving petrels,
P<0.05; common diving petrels, P<0.01). All the tested
thin-billed prions preferred the maze's arm leading to their own burrow
(binomial test, own nest vs neighbour's nest,
P<0.05).

Discussion

These data suggest that two species of diving petrels are able to recognise
the scent of the home burrow. Both common diving petrels and South-Georgian
diving petrels significantly preferred the arm of the maze that contained nest
material from their own burrow over material from a conspecific. This is the
first experimental evidence demonstrating that at least two species of diving
petrels have a functioning sense of smell. We also tested thin-billed prions,
a species that is attracted to food-related odours
(Nevitt et al., 1995) and uses
olfaction in nest recognition (Bonadonna
and Bretagnolle, 2002). Our results indicate that this species
also uses odours to identify the home nest.

While a significant number of diving petrels were attracted to the scent of
their particular nests, many failed to choose an arm of the maze. This failure
to choose may reflect that diving petrels were more stressed by the
manipulation than thin-billed prions. When placed in the maze, most diving
petrels tended to huddle in a corner of the central chamber without moving,
whereas prions tended to always choose an arm. We have noted similar
behaviours when walking in the colony at night. When startled, diving petrels
tend to freeze while thin-billed prions and other larger burrowing petrels
tend to run away (F. Bonadonna, personal observation). This behaviour may
reflect different adaptations to avoid predation in colonies
(Mougeot et al., 1998;
Thoresen, 1969;
Payne and Prince, 1979). Since
diving petrels are darker in colour than thin-billed prions, they may be
better hidden from skuas if they remain immobile. Thus, a tendency to freeze
under stressful conditions may account for the differences recorded in the
percentage of no-choice between diving petrels and other burrowing petrels
that have been tested in this manner (Bonadonna et al.,
2003,
in press).

In terms of their natural history, diving petrels may be particularly well
adapted to finding their nests using their sense of smell. Diving petrels
commonly feed along the outer coasts of Kerguelen Island
(Bocher et al., 2000) and
return to the burrow for incubating shifts or to provision chicks. Arriving at
the burrow involves first locating the specific island within the bay, then
the general region of the island and then the specific burrow. A returning
bird could use a variety of other cues to assist with finding the burrow.
These include visual, auditory and spatial cues, such as knowledge of the
local topography. Although we do not know how much a bird relies upon each of
these sensory modalities, we believe that olfaction plays a large role at
close proximity to the burrow. Once an adult lands on the ground, they are at
high risk of predation and thus it is essential to find the burrow quickly.
Because survival depends on finding the burrow, diving petrels probably do not
use visual cues as the primary method to locate burrows. On Mayes and Verte
Islands, where the present study was conducted, for example, diving petrel
burrows are often located in areas that contain large amounts of the plant
Acaena magellanica (Rosaceae). These plants blanket the ground and
cover the entrance to many burrows (F. Bonadonna, personal observation). It is
unlikely that diving petrels are able to locate these burrows using visual
cues, particularly since birds return to the colonies at night. Behavioural
and anatomical data further suggest that these birds have relatively poor
night vision (Brooke, 1989).
Indeed, researchers working in the colonies at night are often hit by flying
diving petrels, suggesting that diving petrels may be blind to objects even as
large as humans (F. Bonadonna, personal observation).

Alternatively, returning diving petrels could use acoustic information to
locate burrows, but we believe this to be unlikely. Although diving petrels
are vocal in their burrows at night, it is not known if they call to each
other or if mates use calls to guide returning partners to the burrow.
Furthermore, when depleted of reserves, incubating diving petrels will often
leave the burrow to forage before their mate returns
(Chaurand and Weimerskirch,
1994; Warham,
1996). In this case, the incoming adult must locate an empty,
quiet nest. Relying solely upon acoustic information, therefore, may not allow
the bird to successfully find the burrow in an efficient fashion.
Additionally, since the burrows were empty during our experiments, birds could
not use acoustic information to make their choices.

Another possible method for arriving at the home burrow could be the use of
topological information, as suggested by Brooke
(1989). Evidence from our
experiment with South-Georgian diving petrels, however, suggests that they can
recognize the burrow even when its location has been manipulated. In this
case, birds were not being tested on their ability to return to their burrow
but their ability to go towards a metal dish located away from their burrow
that contained their specific burrow material. Other studies suggest that
positional cues alone are not enough for a petrel to find its burrow. For
example, in previous experiments, blue petrels and Antarctic prions were
tested with the arms of the maze pointing towards the burrows, but the arms
were closed off at the end. This prevented air from passing between the burrow
and the maze. In these studies, birds were unable to recognise home burrows,
suggesting that positional cues were not involved in burrow recognition
(Bonadonna et al., 2003,
in press). While these
experiments do not rule out the possibility that positional cues are used
under natural conditions, our experiments suggest that olfactory cues are
sufficient for identifying the home nest.

Taken together, these results suggest that diving petrels are able to
recognise an olfactory signature of their own nest, suggesting that
pheromonal, individual odour recognition or the ability to recognise a burrow
odour is well developed in these species. Further experiments are needed to
clarify the nature of this olfactory signature.

ACKNOWLEDGEMENTS

We are especially indebted to the Institut Polaire Français Paul
Emile Victor (IPEV) for financial and logistical support. Ethical Committee
from IPEV approved the study. David Pinaud prepared
Fig. 1. Dr Richard Van Buskirk
and Rebecca Kihslinger reviewed the manuscript and made helpful comments.
G.B.C. and G.A.N. were supported by OPP 9814326 to G.A.N. We are grateful to
an anonymous referee for valuable comments on an earlier draft of the
manuscript.